1. Field of the Invention
This invention pertains to a novel choke assembly which is particularly useful in reducing pressurized fluids from a high pressure to atmospheric pressure. One embodiment of the invention is particularly useful in combination with a slurry concentrator in which it is necessary to reduce the pressure of the return fluids to atmospheric pressure before the fluids can be recycled.
2. Description of the Prior Art
A wide variety of mechanical devices have been developed for depressurizing fluids (liquids and/or gases). Such fluids have normally been pressurized initially to accomplish a particular chemical or physical feat (e.g. a high pressure chemical reaction, a hydraulic fracturing of a subterranean formation, etc.) or they have been incidentially pressurized. In any event, the pressure generally has to be relieved to make the operation useful. A variety of mechanical devices have been developed to relieve or depressurize fluids. Some of these mechanical devices and theoretical discussions on how they work are described, for example, in handbooks by W. L. McCabe et al., Unit Operations of Chemical Engineering, 3rd Edition, McGraw-Hill, Inc. (1976) and by R. H. Perry et al., Chemical Engineers Handbook, 5th Edition, McGraw-Hill Book Company (1973) and in the technical/patent literature. The prior art devices have included orifices, nozzles, short tubes, etc. These devices, of course, have been designed to regulate or measure the pressures or volumes of the constantly flowing fluids where the volume throughput is regulated primarily by the pressure; such devices are designated herein as "fixed rate" choke devices (as opposed to valve systems which are variable rate in the sense that the volume throughput is determined by opening or closing a valve at substantially constant pressure).
Chokes have been widely used in the oilfield to control the flow rate of high pressure fluids (liquids and/or gases) issuing from the wells. These chokes are usually centered in the flow line so that the velocity decays to a point where the fluid emitted no longer erodes the wall of the piping. Several feet of pipe may be required before the velocity of the fluid is reduced to a point where it is no longer destructive. The configuration of the choke and the minimum length of pipe depends, of course, on the pipe diameter, pressures incurred, the presence or absence of abrasive particles (e.g. sand or bauxite), etc. Another method used in the oilfield has been to direct the pressurized stream of fluid against a sacrificial plug or target (usually made of steel) that is placed in the flow line. This is a moderately effective system, but it represents a brute force type of approach that can result in destruction of the system because of vibration, etc.
Similar problems have been encountered with attempts to control the volume of fluid where there is a pressure differential. Valve Concepts International advertised a valve that is said to be useful at high pressures. This valve appears to consist of a sliding gate circumscribing a cylindrical surface having concentric holes (ports) positioned directly opposite one another in the seat nozzle. The position of the ports directs the flow upon itself and contains the energy, according to the advertisement, and thereby protects the valve body. The volume of fluid flowing through the valve is variable and is governed by the positioning of the gate relative to the series of ports. This valve differs from the present choke system in that it contains a multiplicity of ports rather than two and has a variable volume rather than a fixed or steady state of flow.